1. Field of the Invention
The present invention relates to sheet conveying devices and image forming apparatuses including the sheet conveying devices.
2. Description of the Related Art
Inkjet image forming apparatuses using full-line print heads capable of performing high-speed, high-quality printing on sheets are known in the art. In image forming apparatuses of this type, sheet conveying devices are commonly used in which conductive electrodes charge a conveyor belt for conveying a sheet with electricity so that an electrostatic force is generated and the sheet is thereby retained and conveyed by the conveyor belt.
An example of a known sheet conveying device will be described below with reference to FIGS. 3, 4, and 6.
FIG. 3 is a plan view showing the construction of a conveyer belt, and FIG. 4 is a sectional view of the conveyer belt shown in FIG. 3 cut along line IVxe2x80x94IV. FIG. 6 is a side view showing the construction of a known sheet conveying device.
A known sheet conveying device includes a sheet-retaining unit 36 which serves as an electrode unit for retaining a sheet, a conveyor belt 31 which conveys the sheet while retaining it, and an electricity-supplying unit J which applies a voltage to the sheet-retaining unit 36 to generate an attractive force.
The conveyor belt 31 is an endless belt driven by a driving roller 34 and is disposed around a conveying roller 32 and a pressure roller 35 which are driven rollers. A belt motor (not shown) serves as a driving source of the driving roller 34. The conveyor belt 31 includes the sheet-retaining unit 36, a base layer 36c, and a surface layer 36d. The sheet-retaining unit 36 is formed of electrode plates 36a and electrode plates 36b formed of a conductive metal, electricity-receiving members 36e1 disposed at an end of the electrode plates 36a, and electricity-receiving members 36e2 disposed at an end of the electrode plates 36b. 
FIG. 3 is a plan view of the conveyor belt 31. As shown in the figure, the electrode plates 36a and the electrode plates 36b are alternately arranged in a comb-like pattern.
More specifically, the electrode plates 36a and the electrode plates 36b extend in a direction crossing a sheet-conveying direction C, that is, the direction in which the conveyor belt 31 conveys a sheet, or in a direction approximately perpendicular to the sheet-conveying direction C. In addition, multiple electrode plates 36a and multiple electrode plates 36b are alternately formed on the conveyor belt 31 in the sheet-conveying direction C. All of the electrode plates 36a and the electrode plates 36b are formed in the same length, and are arranged parallel to each other such that the ends thereof are aligned.
The electricity-receiving members 36e1 formed of a conductive material are disposed at one end of the electrode plates 36a arranged on the conveyor belt 31. In addition, the electricity-receiving members 36e2 formed also of a conductive material are disposed at an end of the electrode plates 36b opposite to the end at which the electricity-receiving members 36e1 are disposed. The thickness of the electricity-receiving members 36e1 and 36e2, that is, the vertical size thereof in FIG. 4, is set larger than the thickness of the electrode plates 36a and 36b. In addition, top surfaces of the surface layer 36d and the electricity-receiving members 36e1 and 36e2 are made approximately even so that the top surfaces of the electricity-receiving members 36e1 and 36e2 face outwards from the top surface of the surface layer 36d (that is, so that the top surfaces of the surface layer 36d and the electricity-receiving members 36e1 and 36e2 are in the same plane). The size of the electricity-receiving members 36e1 and 36e2 in the sheet-conveying direction C is 1 cm, and electricity can be supplied, or eliminated, to/from the electrode plates 36a and 36b via different paths.
In addition, the electrode plates 36a and 36b are protected between the base layer 36c and the surface layer 36d in an area in which an attractive force for retaining the sheet is generated.
The base layer 36c and the surface layer 36d are formed of a synthetic resin such as polyethylene, polyamide, a fluorocarbon resin including polyvinylidene fluoride (PVDF), polycarbonate, and polyimide. In addition, the volume resistivity of the base layer 36c is set in the range of 1012 to 1017 xcexa9cm, and that of the surface layer 36d is set in the range of 109 to 1013 xcexa9cm.
The electricity-receiving members 36e1 and 36e2 are formed of, for example, a conductive synthetic resin containing carbon, silver, a conductive paste containing copper powder, whose volume resistivity is 10xe2x88x921 to 105 xcexa9cm.
In addition, the top surfaces of the surface layer 36d and the electricity-receiving members 36e1 and 36e2 are coated with a fluorocarbon resin, etc., so that water repellency thereof increases.
The known electricity-supplying unit J shown in FIG. 6 includes an electricity-supplying electrode 52xe2x80x2 which extends in the sheet-conveying direction C, a pair of electricity-supplying brushes 51xe2x80x2 disposed at the bottom of the electricity-supplying electrode 52xe2x80x2, and a supporter 53xe2x80x2 which retains the electricity-supplying electrode 52xe2x80x2 and the pair of electricity-supplying brushes 51xe2x80x2.
The pair of electricity-supplying brushes 51xe2x80x2 extend parallel to each other at positions directly above the electricity-receiving members 36e1 and the 36e2 such that they are in contact with the electricity-receiving members 36e1 and 36e2, respectively. One of the electricity-supplying brushes 51xe2x80x2 applies a positive voltage to the electrode plates 36a via the electricity-receiving members 36e1, and the other electricity-supplying brush 51xe2x80x2 applies a negative voltage to the electrode plates 36b via the electricity-receiving members 36e2.
When a voltage is applied to the electrode plates 36a, an electric force is generated in the direction shown by the arrows in FIG. 4, so that electric flux lines are obtained. Then, an attractive force is generated at the top surface of the conveyor belt 31 due to the voltage difference between the electrode plates 36a and the electrode plates 36b, and the sheet is retained on the conveyor belt 31 by the attractive force.
However, in the sheet conveying device which is constructed as shown in FIG. 6, even when a sheet cannot be normally conveyed and no sheet is ready in an image-forming operation, or even when the conveyor belt is stained with ink, etc., and the stain is transferred to the paper, such an abnormal state cannot be detected. Accordingly, there is a risk in that the image-forming operation will be performed even though the sheet is absent, so that the surface of the conveyor belt will be stained. In addition, there is also a risk in that the operation of supplying electricity to the sheet-retaining unit cannot be performed effectively because of the stain on the surface of the conveyor belt, so that the sheet cannot be retained with a sufficient attractive force.
In order to detect the situations in which the sheet is absent or the conveyor belt is stained, a sheet detecting unit and a belt stain detecting unit are both required. However, this leads to an increase in costs.
The present invention can provide a sheet conveying device in which a stain on the surface of a conveyor belt and the presence/absence of a sheet on the conveyor belt can be detected with a simple construction, and can provide an image forming apparatus including the sheet conveying device.
According to the present invention, a sheet conveying device used for conveying a sheet includes an endless conveyor belt which includes an endless conveyor belt which includes an electrode unit for retaining the sheet with an electric force and which rotates while retaining the sheet so as to convey the sheet along a sheet-conveying path; a first electricity-supplying unit which applies a voltage to the electrode unit while the electrode unit passes through a predetermined area due to the rotation of the conveyor belt; a second electricity-supplying unit which is disposed at a predetermined position which is upstream of the predetermined area and downstream of a position where the conveyor belt receives the sheet along the sheet-conveying path, and which applies another voltage to the electrode unit while the electrode unit passes by the predetermined position; a detecting unit which detects a current or a voltage when the second electricity-supplying unit supplies electricity to the electrode unit; and a determining unit which determines the state of the surface of the conveyor belt on the basis of the detected current or voltage.
According to another aspect of the present invention, an image forming apparatus, which forms an image on a sheet by using a print head, includes an endless conveyor belt which includes an electrode unit for retaining the sheet with an electric force and which rotates while retaining the sheet so as to convey the sheet along a sheet-conveying path; a first electricity-supplying unit which applies a voltage to the electrode unit while the electrode unit passes through a predetermined area due to the rotation of the conveyor belt; a print head receiving unit which is disposed close to the first electricity-supplying unit and which receives the print head which forms an image on the sheet in the predetermined area; a second electricity-supplying unit which is disposed at a predetermined position which is upstream of the predetermined area and downstream of a position where the conveyor belt receives the sheet along the sheet-conveying path, and which applies another voltage to the electrode unit while the electrode unit passes by the predetermined position; a detecting unit which detects a current or a voltage when the second electricity-supplying unit supplies electricity to the electrode unit; and a determining unit which determines the state of the surface of the conveyor belt on the basis of the detected current or voltage.
According to yet another aspect of the present invention, a sheet conveying method for conveying a sheet using an endless conveyor belt which includes an electrode unit for retaining the sheet with an electric force and which rotates while retaining the sheet so as to convey the sheet along a sheet-conveying path, includes a first electricity-supplying step for applying a voltage to the electrode unit while the electrode unit passes through a predetermined area due to the rotation of the conveyor belt; a second electricity-supplying step which applies another voltage to the electrode unit while the electrode unit passes by a predetermined position, the predetermined position being upstream of the predetermined area and downstream of a position where the conveyor belt receives the sheet along the sheet-conveying path; a detecting step for detecting a current or a voltage when electricity is supplied to the electrode unit in the second electricity-supplying step; and a determining step which determines the state of the surface of the conveyor belt on the basis of the detected current or voltage.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.